The present invention generally relates to gas generating systems and, more particularly, to gas generating systems for inerting fuel tanks.
The combination of fuel vapor and air existing in the fuel tank ullage (empty space above the fuel) can represent a significant threat. For example, the loss of TWA Flight 800, with all passengers and crew several years ago, was caused by the ignition of the fuel vapor in the central fuel tank. The fuel vapor suddenly reacted with great violence with the oxygen present in the ullage. This led to an explosion and subsequent destruction of the aircraft and death of all on-board. The FAA report concluded that type of accident represents a serious threat and that fuel tank inerting should be used to prevent future events of this type.
Inert gas generating systems have been used to protect against fuel tank explosions by replacing the potentially explosive fuel vapor/air mixture above the fuel in the ullage space of the tanks with an inert gas. Inert gas generating systems that provide nitrogen gas have been described. Nitrogen gas does not support fuel vapor oxidation and represents the current best method for on-board generation of an inert gas. Nitrogen can be separated from air using membrane, pressure swing adsorption (PSA), or temperature swing adsorption (TSA). Although these systems require energy to function, they do provide an adequate approach to fuel tank inerting for some applications.
Inert gas generating systems that provide carbon dioxide have been described. U.S. Pat. No. 3,899,099 discloses a system for providing carbon dioxide to tankers. The described system uses a portion of the flue gas from the ship's boiler. Flue gas is a carbon dioxide-rich gas produced by normal, non-catalytic combustion. The flue gas is scrubbed with sea water to remove sulfur dioxide. The scrubbed flue gas is then used for the cargo tanks. Although the described system can provide an inert gas, it requires a boiler or other non-catalytic combustor for fuel oxidation. The described system is unsuitable for some applications because the combustor has limited temperature and compositional ranges and because the combustor requires large, heavy heat exchangers. Additionally, the described system is impractical for some applications because sea water is required to remove the sulfur dioxide from the flue gas.
An inert gas generating system that does not require sea water to remove the sulfur dioxide is disclosed in Japanese Patent No. JP 11-105784. In the described system, sulfur oxides are removed from the flue gas by a desulfurizer. The desulfurized flue gas is then mixed with high purity nitrogen to provide the inert stream. Although the described system may be more practical than the systems that use aqueous solutions to remove the sulfur compounds, it requires a non-catalytic combustor and a supply of high purity nitrogen.
Other carbon dioxide inerting systems generate the inert gas by catalytic combustion of fuel to form carbon dioxide. Carbon dioxide can be easily generated by reacting a small amount of fuel with oxygen in an air stream, and then removing the water that is co-generated. However, some fuels, such as liquid jet fuel, have the disadvantage of containing a substantial amount of sulfur-containing components. The presence of sulfur compounds causes the conversion of the sulfur moiety into sulfur oxides such as sulfur dioxide and sulfur trioxide. These compounds react with water, which is ubiquitous, to form acidic compounds such as sulfuric acid and sulfurous acid. These acids are generally incompatible with metals such as aluminum and aluminum alloys that are used in the construction of fuel tanks and other related components.
As can be seen, there is a need for improved gas generating systems. An inert gas generating system using jet fuel is needed wherein the problems associated with the presence of sulfur compounds are reduced. A carbon dioxide generating system is needed wherein the generated gas has a reduced concentration of sulfur compounds.